Brake systems for a vehicle often rely on hydraulic fluid to transfer force into torque at the wheels of the vehicle. The force transmitted via hydraulic fluid may be generated directly from pressure imparted on a pedal by a driver's foot. Alternatively, the force transmitted via hydraulic fluid may be generated from, and/or multiplied by, a semi-connected or fully-connected actuator. The actuator may be mechanical (e.g., a vacuum booster) or electrical (e.g., an electric motor).
Upon failure of the hydraulic fluid transfer system (e.g., leak or valve malfunction) or failure of the force generation and/or multiplication means, brake systems can be designed to have a fallback braking mode. Generally, a fallback braking mode is fully actuated by the driver's foot to apply counter-propulsive torque at the wheels of the vehicle.
From the driver's perspective, fallback braking modes generally feel vastly different than the primary braking mode of the vehicle. Primary braking modes call for shorter pedal travel and lower input force than what is generally required in a fallback mode. Primary braking modes also provide a higher vehicle deceleration than fallback modes generally offer. In a fallback mode, a brake pedal has a longer travel and a “spongy” feel for the driver such that pedal depression can seem to have no impact on the deceleration of the vehicle until considerable pedal travel distance and force have been applied. Thus, in the event of a primary brake system failure, the sudden change to a fallback mode results in a potentially frightening situation for the driver as the brake pedal assumes unfamiliar performance characteristics. A driver may interpret the change to a fallback mode as a complete loss of braking power and as a result may not push far and/or hard enough on the brake pedal to impart any counter-propulsive torque whatsoever.
Therefore, it may desirable to provide a less abrupt transition to a full fallback braking mode in the event of failure of the primary braking system. Further, it may be desirable to transition to at least one intermediate backup braking mode that has a shorter pedal travel and provides a stiffer pedal feel than a final, full fallback braking mode. Further, it may be desirable to provide a smooth and/or multi-phase transition to a final fallback braking mode in the event of failure of the primary braking system.